Recently, an attempt to equip a high speed vehicle with a millimeter-wave radar has been made in order to prevent accidents caused by drivers' carelessness or errors of judgment in expressways. This is because the advantages of a millimeter-wave radar described hereinafter are taken notice of. Specifically, a millimeter-wave radar is shorter in wavelength compared with a microwave radar or the like, whereby a transmitting/receiving antenna thereof can be reduced in size (so that a vehicle can be easily loaded with the antenna), the width of an antenna beam can be narrowed (which makes it hard to accept clutter from surrounding objects), and the relative velocity with detected objects by Doppler frequency can be detected with high precision.
As shown in FIG. 4, a conventional vehicle with a millimeter-wave radar 1 is provided with a millimeter-wave transmitting/receiving antenna 2 in a front face of the vehicle 1 at a height h of about 0.7 meters above the ground. The millimeter-wave transmitting/receiving antenna 2 has a center C of an antenna beam directed horizontally or upward (.beta..gtoreq.0.degree., .beta. is not shown) and the narrow width of an antenna beam .theta. so as not to be affected by reflection on a road surface (hereinafter referred to as ground clutter G). A transmitting wave 3a is transmitted forward from a transmitting antenna 2a and a reflected wave 3b from a vehicle which is operating 100 meters ahead, for example, is received by a receiving antenna 2b, so that a distance L between the vehicle 1 and the vehicle operating ahead and a relative velocity V can be detected. A pulse method, a two frequency CW method, a FM-CW method and the like are known as methods for processing the transmitting wave 3a and the reflected wave 3b in the millimeter-wave radar. In addition, frequency analysis methods such as a filter bank, FFT (first Fourier transform) or the like are used in order to detect more than one object at the same time.
The conventional vehicle with the millimeter-wave radar 1 is required not to be influenced by the ground clutter G as described above. On the contrary, vehicles for construction such as dump trucks, wheel loaders or the like operating in construction sites, mines, quarries, and the like are required to accurately detect stationary objects while positively accepting the ground clutter G as illustrated hereinafter.
For example, there are "forward detection" for detecting the presence and the absence of steep downhill slopes (which correspond to stationary objects), and "backward detection" for detecting a hopper or an edge of a cliff (which corresponds to a stationary object) in order to safely and accurately dump loads (earth and sand, or stones and rocks) in the hopper or below the cliff after backing the vehicle 1.
Courses of vehicles operating on roads in construction sites, mines, quarries, and the like are usually fixed. In such courses, research on fleet operation by more than one unmanned vehicle have been enthusiastically conducted. In fleet operation by unmanned vehicles, each vehicle stores course data which were previously obtained by teaching, or course data are given to each vehicle from the outside by various sorts of communication means each time. However, the aforesaid roads have road surfaces in bad condition so that the slip ratio of wheels or crawlers, for example, changes extremely depending on weather (rain or snow), soil quality or the like. As a result, even if the distance covered by a vehicle or velocity V thereof, or the distance L between the vehicle and identified objects (for example, a straight line, a curve, a vehicle operating ahead, an intersection and the like) are obtained after the rotational speed or the like of a wheel or the like is obtained, these values are inaccurate. Therefore, fleet operation can not be achieved only with the course data. Specifically, in unmanned vehicles, in addition to the aforesaid "forward detection" and "backward detection", "side detection" for detecting the edge of a cliff, some objects and the like (which are stationary objects) to prevent the vehicles from falling below the cliff on a curve is also necessary.